In recent years, laser light has been used in various applications; for example, laser light has been used in the cutting and working of metals, and as a light source in photolithographic devices used in semiconductor manufacturing apparatuses. Furthermore, laser light has been used in various types of measuring instruments, and in operations and treatment devices used in surgery, ophthalmology, dentistry, and the like.
However, in the case of ArF excimer laser oscillators, the oscillators are constructed with argon gas, fluorine gas, neon gas, and the like sealed inside the chamber, and these gases must be tightly sealed. Furthermore, since these respective gases must also be loaded into the apparatus and recovered, there is a problem in that the apparatus tends to become large in size and complicated. Moreover, in order to maintain a specified laser light generating performance in an ArF excimer laser oscillator, the internal gases must be periodically replaced, or the apparatus must be periodically overhauled.
Accordingly, it is desirable to use a solid-state laser as a laser light source instead of such an excimer laser. However, the wavelength of the laser light that is emitted from a solid-state laser ranges from the visible region to the infrared region; therefore, this wavelength is too long to be suitable for use, for example, in an inspection device. Accordingly, a method has been developed in which long-wavelength light emitted from such a solid-state laser is used after being converted into short-wavelength ultraviolet light (e.g., an eighth harmonic wave) using nonlinear optical crystals. For example, such a method is described in Japanese Patent Application Laid-Open No. 2001-353176.
An outline of the optical system of such a laser apparatus is shown in FIG. 10. In this figure, the objects indicated by oval shapes are collimator lenses and focusing lenses; a description of these lenses is omitted. Furthermore, p-polarized light is indicated by an arrow symbol, s-polarized light is indicated by symbols showing a dot inside a circle, the fundamental wave is indicated by ω, and n-th harmonic waves are indicated by nω. In this example, fundamental light (wavelength: 1547 nm) emitted from a DFB laser (not shown in the figure) is amplified by an EDFA 51 and converted into p-polarized light, and is then caused to be incident on a second harmonic wave forming optical element (PPLN crystal) 52. A second harmonic wave of p-polarized light is generated from the second harmonic wave forming optical element 52 along with the fundamental wave.
This fundamental wave and second harmonic wave are caused to be incident on a triple wave forming optical element (LBO crystal) 53. A triple wave of s-polarized light is generated along with the fundamental wave and second harmonic wave from the triple wave forming optical element 53. This light is caused to pass through a dichroic mirror 54, so that the light is separated into the fundamental wave and the second harmonic wave/third harmonic wave.
The separated second harmonic wave/third harmonic wave passes through a 2-wavelength waveplate 55; in this case, the second harmonic wave is converted into s-polarized light. The second harmonic wave and third harmonic wave that have both become s-polarized light are caused to be incident on a fifth harmonic wave forming optical element (LBO crystal) 56. A fifth harmonic wave of p-polarized light is generated from the fifth harmonic wave forming optical element 56 along with the second harmonic wave and third harmonic wave.
The second harmonic wave, third harmonic wave and fifth harmonic wave are caused to pass through a dichroic mirror 57, so that the second harmonic wave and fifth harmonic wave are separated. The separated fifth harmonic wave is reflected by a mirror 58, and is subjected to beam shaping by cylindrical lenses 59 and 60. Generally, because of walk-off, the fifth harmonic wave generated from the fifth harmonic wave forming optical element 56 has an elliptical cross-sectional shape, so that the focusing characteristics are poor “as is,” and the wave cannot be used in the next wavelength conversion. Accordingly, this elliptical cross-sectional shape is shaped into a circular shape by the cylindrical lenses 59 and 60.
The second harmonic wave separated by the dichroic mirror 57 is converted into p-polarized light by passing through a ½-waveplate 61, and is reflected by a mirror 62. This light is then placed on the same optical path as the fifth harmonic wave described above by a dichroic mirror 63. The dichroic mirror 63 is a mirror that allows the second harmonic wave to pass through, but reflects the fifth harmonic wave. This second harmonic wave and fifth harmonic wave are caused to be incident on a seventh harmonic wave forming optical element (CLBO crystal) 64. A seventh harmonic wave of s-polarized light is generated from the seventh harmonic wave forming optical element 64 along with the second harmonic wave and fifth harmonic wave. Because of walk-off, this seventh harmonic wave also has an elliptical cross-sectional shape, and therefore has poor focusing characteristics “as is,” so that this wave cannot be used in the next wavelength conversion. Accordingly, this elliptical cross-sectional shape is shaped into a circular shape by means of cylindrical lenses 65 and 66.
Meanwhile, the fundamental wave separated by the dichroic mirror 54 is reflected by a mirror 67, and is converted into s-polarized light by passing through a ½-waveplate 68. This light is then placed on the same optical path as the seventh harmonic wave by a dichroic mirror 69. The dichroic mirror 69 is a mirror that allows the fundamental wave to pass through, and reflects the seventh harmonic wave.
This fundamental wave and seventh harmonic wave are caused to be incident on an eighth harmonic wave forming optical element (CLBO crystal) 70. An eighth harmonic wave of p-polarized light is generated from the eighth harmonic wave forming optical element 70 along with the fundamental wave and seventh harmonic wave.
However, in the optical system shown in FIG. 10, the following problems arise: namely, the optical elements that are used are numerous and complicated; furthermore, a dichroic mirror 69 used to combine the fundamental wave and seventh harmonic wave is required. In cases where an eighth harmonic wave with a wavelength of 193 nm is formed, the wavelength of the seventh harmonic wave is 221 nm. In the case of such deep ultraviolet light, general dichroic mirrors show problems in terms of durability. Moreover, an adjustment is needed for the purpose of superimposing the fundamental wave and seventh harmonic wave by means of the dichroic mirror 69, and this work is difficult.